The old town, within what used to be a surrounding fortress wall and mostly still intact, is a fairly compact area, easy walking except for some hills. That is, you can easily walk around to scout the place out to determine what you're interested in in just few hours. It's what you'd want to do there anyway. I have seen some nighttime photos of the place (sorry, no link but maybe search on pbase for Québec), and it's well suited for that, especially if you get a fresh snowfall. Québec is normally cooler than Montréal, say, so it would be unusual not to have snow on the ground in March, but whether you'd have a fresh snowfall is down to luck.
The lower town and the port area has seen a resurgence in the 20 years, since I lived there one summer. It's as interesting as the old town above, but still an easy walk away, although a steep climb to get back up top.
Btw, it's a pain to drive and park in that area, much better to walk. When we visit these days, we get a room in a hotel or B&B near downtown, and leave the car there. Having said that, much of the old town is residential, which means there are parked cars all over the place, ruining some lines of sight for the buildings. You need to scout around to find what you like.
If you google "Bonjour Québec", you will be taken to the main tourism site for Québec, very useful. You won't have any trouble getting by, even if you don't speak French.